This invention relates to a method and apparatus to pull cable and/or to measure the length of a cable-retaining structure, such as an underground conduit or overhead support line, incident to the installation of a cable. More particularly, the present invention relates to using a tape or web comprised of warp and weft threads, preferably with indicia thereon to indicate the length of the cable-retaining structure along which the web extends. If desired, the tape includes conductors for transmitting a tension signal.
In the placement of underground cables, particularly telephone communication cables and electric utility cables, it is desired, and usually necessary, to measure the length of a particular duct in which the cable is placed or overhead line which is to be used for supporting the cable. Sometimes, for example, a cable is provided with connectors prior to its placement in the duct or onto an overhead line. A measurement of the duct length or length of overhead line is first necessary to procure the correct length of preconnecterized cable. It is also necessary to measure the length of a duct or support line when placing a cable comprised of optical fibers for procuring and installing the cable. Careful and cautious handling of a cable made up of optical fibers is necessary. The optical fibers of the cable are particularly susceptible to fatigue fractures that will occur some time after a tension stress excursion beyond the yield point of the optical fiber material. Loss of integrity of the fibers may not immediately occur even though the tension on one or more of the fibers in a bundle exceeds the yield point of the material. Because of this phenomenon, tensioning of a cable comprised of optical fibers must be controlled with greater care during pulling in a conduit or otherwise installing procedures than is usually necessary when installing a cable comprised of metal conductors. As a necessary incident to controlling tension on the cable, it is important that the cable or line which is used to pull the cable possess sufficient strength and a minimum of elongation so that temporary increases and decreases of resistance during the cable placing operation will not result in even momentary stress excursions on the cable.
In the past, the length of a cable duct was measured using a tape comprised of, for example, a weftless web of parallel fibers adhered together by an adhesive. This tape does not possess sufficient strength to permit its use for pulling a cable. Therefore, after measuring the duct length, the tape was used to introduce a pull line in the duct for the actual cable pulling operation. One tape which has been manufactured in the past is made of strands of polyamide resin which possess the strength and chemical resistance necessary to pull a cable in a duct, but unfortunately, the weftless tape is manufactured by adhering parallel threads of polyamide resin with glue or resin. When this occurs, the tape lacks sufficient strength to even permit its use for pulling a winch line into the duct. The tape has almost no further value for accomplishing further procedures to install a cable. However, sometimes the tape can be used to install a nylon rope which is then used to install a wire line which is, in turn, used to install a winch line in a duct to accomplish the cable placement operation. In addition to the foregoing, I have discovered that the use of resins or glues used in the manufacture of such tapes is detrimental to the duct system because the resin or glue adheres to the surface of the duct, thus impeding and sometimes preventing subsequent cable pulling operations.
A minimal elongation of a tape or line under a pull force is important for achieving accurate measurements of a duct. For example, a nylon rope can stretch to over 40% of its free length before breaking. Moreover, when the pull line is used for the cable placement operations, the strength of the pull line must be adequately large so that if exceeded, the pull line will not shear with a dangerous backlash force.